Oscar is the kind of dog who wants everyone to think he is "the big man on the campus". In the wild, Oscar would be the Alpha in his pack.
Some dogs just seem to be more naturally dominant than others. You can see that when grooming time comes around, he is going to let you know how he feels about it. You might even find that he becomes a little on the aggressive side when you approach him with brush in hand. You could hitch him to the grooming table, put a muzzle on him, even sedate him, but this doesn't address the real problem. You need to teach him to become relaxed and calm during grooming.
Although Oscar might be a bit on the dominant side, it doesn't mean he should be allowed to get away with it. It could simply be that he has had a bad grooming experience at some time in the past. It becomes your job to teach him to overcome his fears and anxieties so that he can relax and enjoy his grooming.
Although Oscar might think he is in charge--and in the wild, he may well be--it is your job to make him understand who the Alpha leader in the pack. With a little hard work and practice, Oscar will soon learn to love being groomed and that this is a great time for the two of you to bond.